Regional Differences in Troponin I Isoform Switching during Rat Heart Development

Abstract

Expression of cardiac troponin I (TnI cardiac) and slow skeletal troponin I (TnI slow) genes was analyzed at the mRNA and protein level in the developing rat heart. TnI slow mRNA was detectable by in situ hybridization in the embryonic cardiac tube as early as the 13-somite stage (Embryonic Day 10). In contrast. TnI cardiac transcripts were first detected in the primordial atrium and ventricle of 11-day-old embryos, but were absent in the outflow tract region. TnI slow mRNA levels decreased after birth in atria and later in ventricles but persisted even in adult life in myocytes of the conduction system. TnI slow protein was detected by specific antibodies in atrial myocytes beginning from Embryonic Day 11; in contrast, ventricular myocytes were unreactive until Embryonic Day 18. Western blot analysis of 16-day-old fetal hearts confirmed the expression of TnI cardiac in atrial but not in ventricular myocardium. Slot blot analysis showed that at this stage equivalent amounts of TnI slow and TnI cardiac mRNAs are expressed in atria and ventricles. Similar differences in the expression of TnI slow and TnI cardiac mRNAs and proteins were observed in cultures of embryonic atrial and ventricular myocytes. The results suggest serial rather than simultaneous activation of TnI slow and TnI cardiac genes and they show that different regions of the developing heart differ in their patterns of TnI cardiac expression due to the operation of distinct mechanisms that separately affect the accumulation of TnI cardiac mRNA and protein.